Effective functioning in natural environments demands the integration of input from different modalities such as vision, audition, and touch. For example, in face-to-face communication, combining visual and auditory information leads to better discrimination of speech. Naturally, the neural systems underlying such audiovisual integration have received considerable attention. As functional magnetic resonance imaging and electrophysiological studies reveal, the regions in and around the superior temporal sulcus (STS) are implicated in this process. In humans, MEG or EEG studies show a suppression of the evoked audiovisual response relative to the evoked auditory response in electrodes over the superior temporal sulcus / gyrus (STS / STG). Finally, studies have also implicated different frequency bands such as alpha, beta, and gamma in the processing of audiovisual stimuli [1]. It is unclear how these disparate metrics relate to each other in the processing of audiovisual stimuli.

To relate these findings and provide greater insights into the network-level dynamics of the STS during audiovisual integration, we used a macaque model system to analyze the different frequency bands of local field potential (LFP) responses to the auditory and visual components of vocalizations. These vocalizations (like human speech) have a natural time delay between the onset of visible mouth movements and the onset of the voice (the "time-to-voice" or TTV). We show that the LFP responses to faces and voices elicit distinct bands of activity in the theta (4–8 Hz), alpha (8–14 Hz), and gamma (>40 Hz) frequency ranges. Along with single neuron responses, the gamma band activity was greater for face stimuli than voice stimuli. Surprisingly, the opposite was true for the low-frequency bands—auditory responses were of a greater magnitude. Furthermore, gamma band responses in STS were sustained for dynamic faces but not so for voices (the opposite is true for auditory cortex). These data suggest that visual and auditory stimuli are processed in fundamentally different ways in the STS. Finally, we show that the three bands integrate faces and voices differently: theta band activity showed weak multisensory behavior regardless of TTV, the alpha band activity was enhanced for calls with short TTVs but showed little integration for longer TTVs, and finally, the gamma band activity was consistently enhanced for all TTVs. These data demonstrate that LFP activity from the STS can be segregated into distinct frequency bands, which integrate audiovisual communication signals in an independent manner and perhaps reflect the different scales of the network involved in audiovisual integration. Our analysis of field potential responses in the STS [2] along with prior results from our group from auditory cortex [3, 4] provide a better understanding of the dynamics of audiovisual integration in the regions in and around the superior temporal sulcus / gyrus.

1. Senkowski, D., et al., Crossmodal binding through neural coherence: implications for multisensory processing. Trends Neurosci, 2008. 31(8): p. 401 - 409.
2. Chandrasekaran, C. and A.A. Ghazanfar, Different Neural Frequency Bands Integrate Faces and Voices Differently in the Superior Temporal Sulcus. J Neurophysiol, 2009. 101(2): p. 773-788.
3. Ghazanfar, A.A., et al., Multisensory Integration of Dynamic Faces and Voices in Rhesus Monkey Auditory Cortex. J Neurosci, 2005. 25(20): p. 5004-5012.
4. Ghazanfar, A.A., C. Chandrasekaran, and N.K. Logothetis, Interactions between the Superior Temporal Sulcus and Auditory Cortex Mediate Dynamic Face/Voice Integration in Rhesus Monkeys. J. Neurosci., 2008. 28(17): p. 4457-4469.